1. Field of the Invention
This invention relates to a kiln, particularly suited for the sintering of base metal electrode capacitors.
2. Prior Art
Multi-layer ceramic capacitors have been used extensively as miniature high capacitance, high reliability components in a wide variety of electronic products. These capacitors generally comprise alternate layers of conductive metal (electrode) and dielectric, and may be prepared, for example, by alternate layering of an internal electrode-forming paste and a dielectric layer-forming paste, by sheeting, printing or similar techniques, followed by sintering. In the past, such capacitors were commonly formulated with electrode layer materials that could be sintered in air, for example, noble metal electrodes, such as palladium or palladium alloys. More recently, to avoid the high costs of the noble metals, it has become common practice to replace part, or all, of the noble metal with a relatively inexpensive base metal, such as nickel, or a nickel alloy. However, base metal electrodes (BME) capacitors cannot be easily sintered in air since the base metal oxidizes readily. On the other hand, the dielectric material, typically a ceramic, such as barium titanate cannot be sintered in a strong reducing atmosphere. Thus, a balance must be struck wherein no, or minimal, oxidation of the metal electrode material occurs and no, or minimal, reduction of the dielectric materials occurs. A typical firing cycle will include a sintering step, using a moderately reducing atmosphere, such as, a moist mixture of hydrogen and nitrogen ("forming gas"), followed by a re-oxidation step in which the temperature and oxygen partial pressure are adjusted to re-introduce oxygen into the now partially reduced dielectric while not excessively oxidizing the now sintered electrode. To accomplish this, both the sintering and the re-oxidation atmospheres require narrowly defined oxygen partial pressures, even distribution of the heated gas, and closely controlled temperatures. The kiln must be capable of meeting these requirements through the use of appropriate materials of construction, and a design that provides a carefully controlled flow rate, an even distribution of gas, and a carefully controlled temperature.
U.S. Pat. Nos. 4,241,378 and 4,097,911 disclose a method of making base metal electrode capacitors comprising, for example, alternating layers of nickel metal electrode and barium titanate dielectric, wherein the capacitors are fired in an atmosphere having a lowered partial pressure of oxygen followed by a re-oxidation step.
U.S. Pat. No. 4,517,155 discloses a method for applying copper base metal terminations on multi electrode ceramic capacitors by applying copper glass frit metalizations to the ends of a ceramic capacitor and firing in an atmosphere of nitrogen containing a controlled partial pressure of oxygen.
U.S. Pat. No. 3,414,661 discloses a high temperature furnace wherein the material(s) to be heated are placed on a platform or pedestal that can be moved through various temperature zones. The furnace may be operated at high temperatures, for example, as high as 3000.degree.C in oxidizing atmospheres. The use of neutral or reducing atmospheres at high temperatures is also disclosed.
U.S. Pat. No. 3,619,466 discloses a high temperature electrically heated cylindrical radiation furnace having an adjustably sized heating zone formed by a plurality of arcuate heat-reflecting shields. During operation, gas is distributed through a gas diffuser plate.
U.S. Pat. Nos. 5,559,826 and 5,703,901 disclose a calcination furnace, electrically heated, optionally in combination with combustion heating. The furnace contains an adjustably-sized chamber for the placement of trays holding the units to be processed. Means are provided to control the circulation of the furnace atmosphere as well as the concentration of ingredients therein.